1. Field of the Invention
The present invention generally concerns the reading of information from optical disks, and particularly concerns the parallel reading of one-dimensional holograms that are radially arrayed upon the surface of an optical disk at a high data transfer bandwidth.
2. Background of the Invention
Current secondary digital data storage systems, or memories, have low transfer rates relative to modern computer Central Processor Unit (CPU) processing speeds. Reference B. Robinson, "Grand challenge to Supercomputing," Electronic Engineering Times, 18 Sep. 1989. For memory intensive applications, this creates a performance bottleneck because the memory forces the CPU to wait for data.
Solid state disk drives, with storage capacities of typically 100 Mbytes, typically provide data transfer bandwidths no better than 10 Mbytes/second. Reference L. Curran, "Wafer scale integration arrives in disk form," Electronic Design, 26 Oct. 1989. Although projected developments in main memory technologies such as Static Random Access Memory (SRAM) and Dynamic Random Access Memory (DRAM) could provide bandwidths of 100 Mbytes/second, the capacity of these memories will likely remain severely limited (e.g., 1-10 Mbytes). Reference H. E. Maes et al. ,"Trends in semiconductor memories," Micro Electronics 20, pp 9-57, 1989; and S. Hunter, et al., "Potentials of two-photon based 3-D memories for high performance computing," Applied Optics 29, pp 2058-2066, May, 1990.
Optical disks are good candidates for secondary storage. They combine a high capacity (900 Mbytes for a 51/4" diameter disk), low cost ($1/Mbyte) and robustness (absence head crashes). There are three limitations for high speed operation of optical disk systems: the tracking, the focusing and the addressing functions. All these functions presently require mechanical motions of the head, which motions slow down the disk operation. Moreover the available disk technology is bit-serial sequential, only allowing data rates of up to, typically, 1 Mbyte/second.
It has been shown that optical disks can be read in parallel. Several different parallel readout systems have been proposed. Reference K. Kubota, et al., "Holographic disk with high data transfer rate," Applied Optics 19, pp 944-951, March, 1980; also D. Psaltis, et al., "Optical memory disk in information processing," Applied Optics 29, pp 2038-2057, May, 1990; and also J. Rilum and A. Tanguay, "Utilization of optical memory disk for optical information processing," in technical digest, OSA annual meeting 1988, paper M15.
It would be desirable if any or all of the mechanical motions, and the mechanisms, that are necessary to implement the tracking, focusing or addressing functions of an optical disk memory system could be eliminated, or at least simplified. Simplification, reduction in cost, improvement in shock or vibration tolerance, increased reliability, and/or improved accuracy (signal to noise, and lack of drop-out) of an optical disk readout system might each be desirable under certain circumstances--even if the disk data transfer rate and/or storage capacity were to remain the same, or potentially even to diminish. Alternatively, if the data transfer rate from an optical disk were to be increased by one or two orders of magnitude (the present invention will be seen to increase the data transfer bandwidth by more than three orders of magnitude), then this would be desirable under certain circumstances--even if some negative impact might result on other factors such as cost or reliability.
If an optical disk read out system that was simpler, environmentally harder, more reliable, and likely cheaper than existing optical disk systems, while offering a greatly enhanced data transfer rate, could be realized, then such a system would be interesting, and would seemingly be desirable. If such a new system could be implemented at low cost and with low risk using standard, presently commercially available components (as the system of the present invention will be seen to be), then such a system would be very interesting, and would seemingly be very desirable.
Moreover, a potentially new optical disk system with a "breakthrough" level of performance increase in its data transfer rate would not merely, by virtue of being faster, (i) contribute to the overall throughput of an existing computer data processing system, or (ii) be more generally usable with faster, "super", computers. Instead, such a new system--coupling the existing large capacities, low costs, and reliability of optical disk with a data transfer rate that is typically two orders of magnitude better than semiconductor memories--might render practical certain long-sought functions, and might make possible wholly new functions.
In this regard, it is particularly noted that an associative memory, or content addressable memory, of a practically valuable size and throughput (measured in bit-operations/second, as opposed to bits/second) has long been sought. Reference, for example, T. Kohonen, Self Organization and Associative Memory, Springer Verlag, 1984. The motionless head optical disk readout system of the present invention, and logical extensions thereof including extensions into a third media dimension, will be seen to be the basis of the OPTOELECTRONIC ASSOCIATIVE MEMORY USING PARALLEL-READOUT OPTICAL DISK STORAGE that is the subject of the companion patent application.
Because the motionless head optical disk readout system of the present invention will be seen to hold so much data (940 Mbytes in the preferred embodiment) and to realize its high data transfer rate (about 1.1 Gbytes/sec. in the preferred embodiment) by reading such data in very large words (16,384 bits/word, or a 128.times.128 pixel bit "image" in the preferred embodiment) at high rates (560,000 words/sec.); because one preferred use of the read words is to contain image data (128.times.128 pixels per word); and because the read out system is optically imaging, each retrieved word is sometimes called an "image". It will be understood by a practitioner of the art of high performance computer memory design that the size of each retrieved "word" from such an optical disk system in accordance with the present invention so greatly transcends the typical "word" widths of some 16, 32 or 64 bits that are commonly used in the CPU's of computers that such a optical memory "word" may, on occasion, be called something else. Namely, the very large word is typically called an "image". Use of the word "image"--such as is in "image processing" or "image recognition"--usefully suggests the advanced uses to which a very high data transfer bandwidth memory may be put. It will therefore further be understood by a practitioner of the art of high performance computer memory design that the word "image" is a term of art meaning a word, typically consisting of very large number of typically binary bits, that is retrieved from a high performance memory during each read cycle. The word "image" does not simply mean a pictorial or optical reproduction, nor, invariably, the data that permits of such a reproduction.